The present invention relates to power conditioning circuits and, in particular, to circuits accepting either an alternating or direct current input and producing a reduced voltage output.
There are many situations in which one would like to employ relatively low voltage devices. However, power is generally supplied at 110-120 volts or higher levels at frequencies of either 50 or 60 Hz. Many schemes exist for transforming this voltage to lower voltages which are useful in certain specific applications. One of the techniques employed for such voltage conversion is the use of step down transformers. However, transformers tend to be expensive, bulky, heavy and tend to exhibit the efficiency-diminishing effects of core loss. Other means for effectively reducing voltage include the use of such devices as silicon controlled rectifiers (SCRs). However, these devices can result in undesirably high levels of radiated electromagnetic noise, low power factor, and they often produce an output voltage waveform significantly different in form from the input waveform. In yet another scheme, an auxiliary capacitor is connected across a main capacitor in series with the load during a selected portion of the source waveform cycle to provide and control a low voltage across a resistive load. Such a scheme is described, for example, in U.S. patent application Ser. No. 379,393, filed on May 18, 1982, in the names of William P. Kornrumpf and Paul T. Cote, and assigned to the same assignee as the instant invention. However, that circuit exhibits a relatively low power factor and draws correspondingly high levels of current in certain applications. Accordingly, it is seen that it is desirable to be able to construct an alternating current voltage step down power conditioning circuit having the high power factor advantages of transformer circuits, without the necessity of employing bulky, heavy or costly transformer devices. The present invention fulfills these requirements and is additionally able to accept d.c. power input of either polarity and still function to produce an adjustable output voltage less than or substantially equal to the input voltage. The power conditioning circuit of the present invention is particularly useful in powering low voltage incandescent lamps. However, the circuit is equally applicable to other electrical devices including heaters and motor-driven appliances. Additionally, the circuit of the present invention not only provides a step down in the line voltage, but also provides a means for varying the output voltage and for such purposes as continuous lamp dimming at high power factor and with high power conversion efficiency.